The invention relates generally to optical sensors and more particularly to fiber optic and other optical waveguide sensors.
Research and Development on sensors for the detection and quantification of particular chemical species or classes of chemical compounds, is one of the fastest growing technical fields. Yet for all of the effort being expanded, the program is very much fragmented. Each physical or chemical measurement to be made becomes a special project because there is no common ground on which a basic foundation can be laid. Fiber optic chemical sensors (FOCS), CHEMFETs, piezoelectric crystals, semiconductors, etc. require different chemistries (or chemical composition) and sensor design for each specific measurement that is to be made. This means that each time a new target is identified, the research and development effort must start anew.
A new type of sensor as described in U.S. patent application Ser. No. 046,986 has been developed which is adaptable to the analysis of several target molecules or classes of compounds. This would eliminate the need for a large R&D effort for each species to be measured. It is based on the postulation that a custom fiber optic can be made which itself is the sensor. This is done by taking advantage of the relationship between the refractive indices of the core and the clad and how these can be exploited to sense and monitor a species of known refractive index. In this situation, when the target compound interacts with the clad, the optical properties of the sensor change, thus affecting light propagation through the fiber. The light transmission properties which are related to this alteration can be directly related to the analytical information being sought.
Light transmission through a fiber optic is an evanescent wave. If the refractive indexes of air, the core and the clad are N.sub.0, N.sub.1 and N.sub.2 respectively, then the angle at which the light enters the clad, A.sub.c, is defined as: EQU A.sub.c =sin.sup.-1 N.sub.2 /N.sub.1 ( 1)
and the numerical aperture, NA, is dependent on the entrance angle of the light, A.sub.m through the relationship: EQU sinA.sub.m =NA=(N.sub.1.sup.2 -N.sub.2.sup.2).sup.1/2 /N.sub.0 ( 2)
Most important, as to the core of an optical fiber, is that for light to propagate along the fiber, it is required that N.sub.1 &gt;N.sub.2.
Areas where the refractive index based sensors should have commercial application include: (i) ice detection, (ii) process control and regulation, (iii) pollution and environmental monitoring, (iv) discovering leaks in pipes and storage tanks and (v) quality inspection of liquids and gases.
The U.S. Air Force, for one, is interested in cooling tower ice detection techniques. Present sensor technology is not compatible with the Air Force's needs for ice detection. In particular, size, specificity, sensitivity and lifetime are requisites that cannot be met with existing devices. Therefore, a completely new, fiber optic sensor technology is desired which has the potential to meet all of the Air Force's requirements. It is also desirable to apply the principles of an ice sensor to detect other chemical species.